Methylated DNA sequences in genomic imprinting.

Genomic imprinting is a special form of epigenetic system that determines the parent-of-origin-specific, or monoallelic, expression of a small number of genes, termed "imprinted" genes. Considerable sequence and methylation analysis of imprinted genes has revealed a common theme: Regions of allele-specific methylation inherited from the gametes, or primary differentially methylated regions (DMRs), are associated with CpG islands and repeat elements, and this overall structure suggests functional significance. For at least three imprinted genes the sequence of the primary DMR constitutes an element able to regulate gene activity in cis--a chromatin insulator and a promoter of an antisense transcript. In these cases the unique feature of imprinting appears to be in the ability to switch the regulatory capacity of these elements on or off by the absence or presence of inherited methylation. Increasing evidence therefore suggests that genomic imprinting for at least some genes constitutes the regulation of gene regulatory elements by methylation. An important challenge now is to determine how the differential methylation of primary DMR sequences is established in the germ line. If methylation is the primary imprint, then the processes establishing it are the primary imprinting mechanisms. Trans-acting factors that are expressed in one sex of germ line and not the other are likely to be involved, and their ability to methylate may be mediated through repeat elements associated with the sequence of primary DMRs.